kpagecount: add slab page checking because _mapcount is in a union

Add a PageSlab() check before adding the _mapcount value to /kpagecount.
page->_mapcount is in a union with the SLAB structure so for pages
controlled by SLAB, page_mapcount() returns nonsense.

Signed-off-by: Petr Holasek <[e-mail]>
Cc: Wu Fengguang <[e-mail]>
Cc: Matt Mackall <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/page.c

···
 			ppage = pfn_to_page(pfn);
 		else
 			ppage = NULL;
-		if (!ppage)
+		if (!ppage || PageSlab(ppage))
 			pcount = 0;
 		else
 			pcount = page_mapcount(ppage);